The present invention relates generally to a method and apparatus for accurately measuring minute displacements of a movable object and, more particularly, to a simple, yet precise, optical method and apparatus for providing such displacement measurements.
There is a great need in many applications to monitor the positions of a movable object relative to a desired position or positions. For example, it is desirable in disk drive servo systems to accurately detect the positions of a rotatable read/write head arm, relative to desired, known positions, so that the head arm can be moved into alignment with desired radial track locations on a disk to enable the read/write head to read from and write to such locations. Position sensing devices are devices aimed at providing such position detection. Displacement sensors are position sensing devices that monitor the positions of a movable object by repeatedly measuring the displacements of the object from a desired location.
Examples of conventional displacement sensors include capacitance gage devices, fiber-optic proximity sensors and optical sensors, such as interferometric sensors used commonly in disk drive servo systems. Many of the prior art displacement sensors either are incapable of achieving the degree of resolution (the smallest measureable displacement) required by certain applications (such as disk drive servo systems) or require expensive and/or complex circuitry and hardware to achieve such resolution. To achieve high resolution, certain displacement sensors require, for example, a precise and powerful laser light source or extremely close proximity between the sensing element and movable object, rendering such devices expensive and difficult to implement. Thus, a balance must be struck between device performance and simplicity.
U.S. Pat. No. 5,315,372 to Tsai describes a prior art disk drive servo system that employs an optical displacement sensor. The Tsai device includes a light source and a photodetector array attached, at spaced-apart locations, to a rotatable master arm, located externally of the disk drive. A reflector is attached to a rotatable read/write head arm at a location between the read/write head and the axis of rotation of the arm. During operation, the master arm first is aligned accurately with a desired radial track location on the disk using an interferometric device. Then, the displacement sensor operates to determine the position of the head arm relative to the master arm so that the head arm can be moved into alignment with the master arm.
The light source of the displacement sensor produces an incident light beam that is reflected by the reflector onto the photodetector array. The position on the photodetector array to which the light beam is reflected depends on the relative radial positions of the head and master arms. Each photodetector element of the array produces an electric signal having an amplitude proportional to the intensity of the received light. The signals produced by the array, thus, represent the relative radial positions of the head and master arms. Processing circuitry receives and decodes the signals output by the array to determine the relative head arm position and accordingly controls a motor to rotate the head arm until it is properly aligned.
The device disclosed in the Tsai patent suffers from a number of drawbacks. While the Tsai device is relatively structurally simple and fairly inexpensive to implement, it is burdensome to operate. The Tsai device requires a pre-knowledge of each master arm and relative head arm position in order to accurately interpret the electric signals produced by the photodetector array. Also, due to the significant spacing between the reflector and the read/write head on the head arm and, due also to the dual axes of rotation of the head arm and master arm, performance accuracy is sacrificed severely. Further, the displacement sensor of Tsai aims to monitor radial movements (those caused by rotations about the axis) of the head arm relative to the master arm. Because the reflector reflects the incident beam directly onto the photodetector, spaced significantly from the reflector, the sensor is sensitive not only to radial movements of the head arm but also to angular movements of the reflector. Thus, the precise angular orientation of the reflector on the head arm is crucial to precise operation. Any angular movements of the reflector with respect to the head arm may cause false measurements to occur.
It accordingly is a general object of the present invention to provide a simple, yet accurate, displacement sensor.
One embodiment of the present invention is directed to a device for measuring the displacement of a movable object. The device includes a stationary light source that produces an incident light beam. A target feature, attached to, or integral with, the object, reflects the incident light beam and forms a first image of the light source in close proximity to the target feature. The first image moves with the target feature. An imaging lens receives the reflected light beam and forms a second image of the light source on a stationary photodetector. The photodetector, spaced from the object, receives the second image and, in response thereto, produces an electric signal having a characteristic (such as amplitude) which is proportional to an instantaneous position on the photodetector of the second image and which represents the position of the object. The target feature includes a curved surface that reflects the light beam such that a small, preferably point-like or line-like first image of the light source is formed near the target feature and is reformed as a second image on the photodetector. As a result, the device of the invention measures only lateral movements of the object and is insensitive to other movements such as the angular orientation of the object.
In one embodiment of the invention, the photodetector includes a spatially arranged photodetector, such as a bi-cell photodetector. In another embodiment of the invention, the photodetector includes a position sensor.
In an embodiment of the invention, the target feature preferably has a radius of curvature within the range of 0.2-0.5 mm.
In an embodiment of the invention, the incident light beam and the reflected light beam form a plane substantially perpendicular to the direction of the motion to be sensed. In that embodiment, an axis formed through a center point of each cell of the bi-cell detector is substantially orthogonal to the plane and substantially parallel to a direction of motion of the object.
Another embodiment of the invention is directed to an optical displacement sensor for use in a disk drive servo system. The sensor monitors the relative radial positions of a read/write head arm and a master arm. The device includes a light source, attached to the master arm, that produces an incident light beam. A target feature, attached to, or integral with, an end of the head arm, reflects the incident light beam and forms a first image of the light source in close proximity to the target feature. The first image moves with the target feature. An imaging lens receives the reflected light beam and forms a second image of the light source on a photodetector. The photodetector, attached to the master arm, receives the second image and, in response thereto, produces an electric signal having an amplitude which is proportional to an instantaneous position on the photodetector of the second image and which represents a relative position of the head arm. The target feature includes a curved surface that reflects the light beam such that a small, preferably point-like or line-like first image of the light source is formed near the target feature and is reformed as a second image on the photodetector. As such, the displacement sensor is sensitive only to lateral movements of the target feature and is insensitive to other movements. Thus, the angular orientation of the target feature on the head arm will not affect measurements and need not be precise.
A further embodiment of the invention is directed to a method of measuring the displacement of a movable object. The method includes the steps of: producing an incident light beam with a stationary light source; reflecting the incident light beam from a curved surface of a target feature to form a first image of the light source in close proximity of the target feature, attached to, or integral with, the object; small, reforming the first image of the light source as a second image thereof onto a stationary photodetector; and producing, with the photodetector, an electric signal having an amplitude which is proportional to each instantaneous position on the photodetector of the second image and which represents a position of the object.
The features and advantages of the present invention will be more readily understood and apparent from the following detailed description of the invention, which should be read in conjunction with the accompanying drawings, and from the claims which are appended to the end of the detailed description.